Revealing the Role of Carbon Layers in Lithium Manganese Iron Phosphate Cathodes: Synergistic Enhancement of Interfacial Charge Transport and Structural Stability

Lithium manganese iron phosphate (LMFP) has attracted considerable interest for its superior energy density compared to LiFePO4. Nonetheless, the practical implementation of LMFP faces challenges due to its naturally poor electrical conductivity and manganese dissolution, which arises from the Jahn–Teller effect. In this study, a comprehensive investigation of the synergistic effects of carbon coating was elucidated, demonstrating simultaneous enhancements in interfacial charge transport and structural stability. Experimental results highlight that precise control over the coating amount is a key strategy for achieving high-performance LMFP. The carbon serves as a physical barrier to avoid direct interaction between the LMFP and the electrolyte, mitigating the corrosion of Mn ions on the material surface caused by acidic components in the electrolyte. Further ex situ XRD analysis demonstrates that optimizing carbon content reduces lattice expansion during redox reactions, thereby improving the material’s structural integrity. Consequently, the optimized sample demonstrates enhanced electrical conductivity and a robust structural framework. It achieves an exceptional discharge capacity of 138 mAh/g at 2 C, along with outstanding capacity retention of 98.4% at 1 C after 300 cycles. This work deepens the research on carbon-coating modification and provides valuable insights into the design of olivine-based cathode materials with high Mn/Fe ratios.